Oil refiners are interested in improving the energy efficiency of atmospheric crude oil distillation. Atmospheric crude oil distillation columns typically separate crude oil into residue, gas oil, distillate, kerosene and naphtha fractions. Atmospheric crude oil distillation units are highly heat integrated, and heat is recovered from the products of the atmospheric distillation column and used to preheat the crude oil feed by indirect heat exchange. The remaining heat that is required for feed heating is usually supplied by sending the preheated crude oil to a fired heater, conventionally termed a crude heater, before it enters the atmospheric distillation column.
Heat can be recovered from the exhaust of a gas turbine engine and used for process heating. Linnhoff and Townsend, “CHEM. ENGR. PROG.,” 72, 78 (1982) described recovery of turbine exhaust heat for process heating. It is also known to those skilled in the art that the exhaust gas from a gas turbine engine contains a significant amount of residual oxygen. This is because gas turbine engines are usually operated with an air flow in large excess of that required by stoichiometry in order to limit the turbine inlet temperature for metallurgical reasons. Because of the residual oxygen content, the turbine exhaust can be secondarily fired with a duct burner or in another furnace. This practice is widely used in heat recovery steam generators placed on gas turbine exhaust streams. Terrible et al., “HYDROCARBON PROCESSING, 43, vol. 78 (Dec. 1999) describe a steam methane reforming process in which the reforming furnace is heated by secondary firing of a gas turbine exhaust gas.
Secondary firing of gas turbine exhaust would seem to be an attractive means of supplying heat to an atmospheric crude oil distillation column. This concept has never been commercially practiced, however, because the availability of gas turbine engines is low relative to the requirements for a crude heater. Aeroderivative gas turbine engines are available only typically in the range of 97% to 99% of the time. Part of the lost time is due to planned outages for maintenance, as the engines require bore scoping once or twice each year, which entails a 24-48 hour shutdown, as well as more major overhauls after 25,000 and 50,000 hours of operation. The remaining down time between 2 and 10 days per year is due to unplanned shutdowns. For example, the GE LM6000 gas turbine engine has an availability of 98.8%, corresponding to an average of 105 hours of down time per year, of which 36 hours are for planned maintenance and 69 hours are for unplanned outages. An atmospheric distillation unit is usually required to run continuously for a period of three to five years, and any interruption in this operation necessarily stops all production in the refinery. Consequently, refiners are reluctant to exploit this energy-saving opportunity if the reliability of the entire refinery is potentially jeopardized.